Cider Hard and Sweet, Ben Watson
Cider: Making, Using & Enjoying Sweet & Hard Cider, Annie Proulx & Lew Nichols
I think that cider apples have enough stuff in them that you don't have to worry about adding nutrient. Also, since cider ferments out at about 5-7% ABV, you aren't straining the yeast that much. The exception might be if you're making a fortified cider like a New England Cider and taking the ABV much above 6%. Then it might be useful to add some nutrient.
It's my experience that cider, even fortified ciders ferment out relatively quickly without needing much help. The real skill in making cider comes from choosing the correct combination of apples and yeast strain, fermenting it at the proper temperature and arresting fermentation or back-sweetening to get the right amount of residual sugar. Also, for competition, it's a hassle to get cider to the bright to brilliant clarity that judges expect.